1. Field of the Invention
The present invention relates to purple membrane preparations having increased holographic diffraction efficiency.
2. The Prior Art
The use of bacteriorhodopsin or bacteriorhodopsin variants as the active component in optical recording media is known. Thus, for example, the article by N. Hampp and C. Brauchle entitled, Bactedorhodopsin and Its Functional Variants: Potential Applications in Modern Optics, published in Organic Chemistry 40, Photochromism, 954-975, Ed. H. Durr, H. Bouas-Laurent, Elsevier 1990, tabulate possible uses of bacteriorhodopsin.
For reasons of greater stability to thermal, chemical and photochemical destruction or degradation, the bacteriorhodopsin and/or its variants as the active component in optical recording media is preferably not employed in the form of the free molecules, but instead in membrane-bound form, i.e., generally in the form of optionally comminuted purple membrane or variants thereof. Variants of purple membrane, i.e., membranes which contain variants of bacteriorhodopsin, can be obtained in a manner known per se with the aid of biotechnological processes, for example, by varying the retinal chromophore by chemical exchange (see, for example, Methoxyretinals in Bacteriorhodopsin. Absorption Maxima, Cis-Trans Isomerization and Retinal Protein Interaction, by W. Gartner, D. Oesterhelt, 1988, Eur. J. Blochem. 176, 641-648) or by modifying the bacterio-opsin molecule (see, for example, D. Oesterheit, G. Krippahl, Ann. Microbiol. (Inst. Pasteur) 134B (1983) 137-150). The characterization of bacteriorhodopsin variants with modified bacterio-opsin is described, for example, in Kinetic Optimization of Bacteriorhodopsin by Aspartic Acid 96 as an Internal Proton Donor, by A. Miller and D. Oesterhelt, Blochim. and Biophys. Acta (1990) , 1020, 57-64.
A promising application of purple membranes is in holography. Holography is taken to mean purely holographic processes and optical processes which involve one or more holographic part-steps.
The advantages of using purple membrane preparations for said purposes lie, in particular, in the favorable absorption range, the high resolution which can be achieved, the large number of possible write/erase cycles, the long shelf life, the high sensitivity and the light fastness. Use in color holography is also possible. This applies to all hologram types which are possible using purple membrane preparations, for example, the embodiments known as B-type holograms, in which photoconversion of the initial stage (=B) of the active component is carried out, or those hologram types in which an intermediate state or photointermediate is subjected to photoconversion (for example, M-type hologram). It is advantageous for this purpose to select an intermediate whose absorption properties differ significantly from those of the initial state. Such an intermediate which can favorably be employed for holographic purposes is available, for example, in the form of the intermediate state of naturally occurring bacteriorhodopsin, frequently known as the M-state.
For holography, purple membranes can be used, for example, in the forms of suspensions or films.
Purple membrane suspensions contain purple membranes in water, i.e., the water content in purple membrane suspensions is greater than the purple membrane content. Purple membrane suspensions can be used as recording materials for information in which only low spatial resolution of the recording material is required and/or long rise times of the holographic diffraction efficiency are sufficient.
Holographic diffraction efficiency is taken to mean the ratio between the intensity of light diffracted at the holographic grating and the intensity of the light incident on the holographic grating. The holographic diffraction efficiency is generally given in percentage. The maximum value which the diffraction efficiency can reach is 100%. The holographic diffraction efficiencies given for the purple membrane preparations according to the invention relate, as is also usual for the data in the prior art, to a wavelength of 632.8 nm. The rise in the holographic diffraction efficiency is the time from commencement of exposure until the maximum diffraction efficiency has been reached. The shorter this time, the more the material is suitable for dynamic applications. The minimum of the theoretically achievable rise time of the holographic diffraction efficiency is about 50 .mu.s for purple membrane preparations. The definition of the holographic diffraction efficiency and a description of the correlation between diffraction efficiency and wavelength is given in Kogelnik, H. (1969), Bell Syst. Tech. J. 48, 2909-2947.
The use of suspensions containing purple membranes as recording material is described by V. Y. Bazhenov et al. in Optical Processing and Computing, in Chapter 4, Biopolymers for Real-Time Optical Processing, 103-144. Although this publication mentions bacteriorhodopsin suspensions having a diffraction efficiency of up to 6%, nothing is stated on the preparation and composition of the suspensions. These suspensions have diffusion-limited rise times of the diffraction efficiency, which results in rise times of the diffraction efficiency of 7 seconds or longer. These rise times of the diffraction efficiency are too slow for use in dynamic holography. Purple membrane suspensions allow resolutions of from 0 to 500 lines/mm to be achieved. The suspensions are therefore unsuitable for use in dynamic holography or for use as high-resolution storage materials, where resolutions up to 5,000 lines/mm are necessary. Neither is it possible to achieve M-type holograms using these suspensions.
This disadvantage is not exhibited by media which contain immobilized purple membranes. In these, the purple membrane, in the form of films or gels embedded in support materials, such as, for example, polymers, is preferably applied to substrates, such as, for example, glass plates or mirrors, or employed in another form which allows a reproducible two-dimensional arrangement.
According to Hampp et al., in Thin Films in Optics (Ed.: T. Tschudi), Proc. SPIE 1125, pp. 2-8, films containing purple membrane are obtained by drying purple membrane suspensions on siliconized glass plates. The optical homogeneity of the films can be increased by employing mixtures of water-soluble polymers with purple membrane suspensions. Examples mentioned of water-soluble polymers are polyvinyl alcohol and polyvinylpyrrolidone. A further method mentioned is the embedding in a matrix by direct polymerization in polyacrylamide gels. No details are given on the production of films containing purple membrane. The films have a water content of 60%. The maximum diffraction efficiency is 0.2% for films containing wild-type purple membrane (BR-WT) and 0.3% for films containing a purple membrane variant in which aspartic acid has been replaced by asparagine in position 96 of the bacteriorhodopsin (BR 326 variant).
The article by N. Hampp and C. Brauchle entitled Bactedorhodopsin and Its Functional Variants: Potential Applications in Modern Optics," published in Studies in Organic Chemistry 40, Photochromism, pp. 954-957, Ed. H. Durr, H. Bouas-Laurent, Elsevier, 1990, describes the use of wild-type bacteriorhodopsin and the above-mentioned position-96 variant of bacteriorhodopsin BR 326 in the area of modern optics and holography. The production process disclosed for films containing bacteriorhodopsin involves drying purple membrane suspensions on a glass substrate or embedding it in a polymer. According to this publication, the purple membrane is employed as purchased or prepared for the film production. No details are given on parameters such as salt concentration or buffer solutions or any assistants. The films described therein have a diffraction efficiency of 1% if WT purple membrane is used or 2% if BR-326 variant is used. Biophys. J. 58, 83 (1990) , pp. 83-93, discloses various properties of films containing wild-type bacteriorhodopsin or the position-96 variant of bacteriorhodopsin BR 326 for films. The films containing purple membrane were obtained by drying an aqueous purple membrane suspension having a pH of from 6 to 7 on silanized glass plates dried in air overnight. The films have a water content of 60%. According to this publication, the purple membrane is adjusted to a pH of from 6 to 7 for the film production. No details are given on parameters such as salt concentrations or buffer solutions or any assistants. The films obtained have a diffraction efficiency of from 0.2 to 0.4%.
A severe disadvantage which hitherto prevented broader application of purple membrane preparations in holography is the low holographic diffraction efficiency which can be achieved by known purple membrane preparations. This low diffraction efficiency makes it necessary, for example, to use strong lasers to detect the stored holographic information in order to obtain in the detection system a signal which can be clearly registered, i.e., enables a good signal/noise ratio to be obtained. Strong lasers have disadvantages in practice. For example, high laser power is expensive, strong lasers are bulky, and the operation of strong lasers requires relatively high safety standards. In addition, a high laser power results in heating of the sample by the undiffracted light passing through the sample, which may, under certain circumstances, result in destruction of the stored information.
In spite of their relatively high diffraction efficiency, known purple membrane suspensions are, as stated, likewise unsuitable for broad applications due to their slow rise time of diffraction efficiency and their low spatial resolution.